This invention relates to a flat cathodoluminescent image display device employing a modulation mask and particularly to a modulation mask employing digital control circuitry.
One form of a flat image display device, to which the present invention relates, includes a multiplicity of cells which may include spaced parallel channels. Each of the cells includes all the necessary components for forming at least a single element of an image display. Typically, each cell includes a line source of electrons which may comprise a source of primary electrons, hereinafter referred to as the cathode, and a multi-dynode electron multiplier open to feedback and of sufficiently high gain to produce regenerative feedback so as to provide a self-sustaining source of electrons. The cell also includes means for modulating the flow of electrons which exit from the multiplier. In addition, the cell includes means for accelerating and focusing the modulated flow of electrons to a cathodoluminescent screen excitable by the accelerated and modulated flow of electrons.
A device of the type utilizing a matrix of cathodoluminescent cells is operated by suitably addressing the cells in a desired sequence, for example as in a typical television scan. In one form of a flat image display device to which the present invention relates, for example one which employs multi-dynode line electron multipliers for generating the required flow of electrons, line scanning is accomplished in the following manner. Accelerating voltages are applied to every dynode in the horizontal line multiplier which is required to produce a flow of electrons and a repelling voltage is applied to at least one dynode of the remaining horizontal line multipliers in order to turn off the flow of electrons in those multipliers. Consequently, each horizontal line can be turned on in any desired sequence.
To be useful as an image display, it is apparent that the output electrons from the line electron sources must be accurately modulated. Gray-scale modulation, that is, a selective gradation of a number of electrons permitted to strike the screen, can be accomplished by use of a modulation mask. The modulation mask includes a metal layer having a plurality of substantially identical slots which are disposed in parallel rows and columns, each slot being associated with a picture element. Each slot is surrounded by a sensing pad insulatingly mounted on the surface of the metal adjacent to the electron multiplier and is also surrounded by a modulating electrode insulatingly mounted on the opposite surface of the metal layer. The sensing pads function as capacitors which are charged uniformly from slot to slot. This uniform charging of the sensing pads assures that the amount of electrons passing through each slot is substantially uniform from slot to slot upon discharge of the pads by the line electron sources. A modulating voltage is applied to each of the modulating electrodes on the opposite surface of the modulation mask. If the modulating voltage varies continuously (i.e., in an analog fashion) as a function of the video signal, then the modulating electrodes, with the modulating voltages applied thereto, will permit a continuously varying (from full charge to zero) amount of electrons to impinge upon the phosphor screen, thus controlling the brightness of the image appearing at that picture element.
It is characteristic of analog modulation of the sensing pad modulation mask that while full brightness uniformity can be achieved simply by holding geometrical tolerances, lower level signals are much more difficult to hold to a desired tolerance level of full brightness of approximately 1% element-to-element. Consequently, while full brightness can be controlled uniformly from element-to-element, displayed information having less than full brightness can vary considerably outside the desired brightness uniformity tolerance. This non-uniformity at lower brightness levels occurring with analog modulation can have a deterimental effect on the overall quality of the video display.